Biophy.

Have you ever wondered why the effects of fluctuations will not affect how our organs work?
How are goosebumps formed and why does it form when we feel cold?
How does cold-blooded animal (ectotherm) deal with heat variation?
why fish dont freeze to become instant sashimi?

These are all due to thermoregulation.

Thermoregulation is one of the ability of an organism which helps to keep its body temperature within certain boundaries, even when temperature of the surrounding varies. This process is known as homeostasis: a dynamic state of stability between an animal's internal environment and its external environment.

Life on earth exists within a narrow range of temperature which is stabilized by the unique properties of water within the bodies of organisms. Enzymes become denatured at temperature extremes and metabolic reactions occur best at certain temperatures. Both endotherms, which control the build-up of heat from aerobic respiration and ectotherms that can thermoregulate, but only the endotherms (birds and mammals) can maintain a stable body temperature by using their nervous, endocrine, respiratory and circulatory systems.

Endotherms : An endotherm is a warm-blooded animal (such as homo sapiens) that generates heat to maintain its body temperature, typically above the temperature of its surroundings.

Ectotherm : An organism that regulates its body temperature largely by exchanging heat with its surroundings.

Thermoregulation is one of the ability of an organism which helps to keep its body temperat

There are two types of thermoregulation that are used by animals:
1. Physiological regulation: This is when an organism changes its physiology(function of organ) to regulate body temperature. For example, our body tends to sweat in order to cool our body down. Another example is when our bodies get cold; it tends to shiver so that the body can create some heat.
2. Behavorial regulation: This is when an organism changes its behavior to changes it body temperature. For example, when your body starts to get hot because of the sun, you may want to find a shade to shield yourself away from the sunlight.

In a healthy individual, the temperature of the body is adjusted by feedback control mechanisms that maintain it nearly constant around (37°C) throughout the day, week, month or year. This thermoregulation is efficiently managed by the central nervous system (CNS) as long as the temperature of the surroundings ranges between 20°C and 54°C .

However, there are some variations of temperature in our body.
1. Variation due to food & drinks intake different type of food intake generate heat or cold within our body cells.
Papaya, Mango, Wine, red-chilies and other spicy items generates heat.
Milk, winter melon, herbal tea, are famous for reducing the body heat.
This happens when the process of digestion and consumption by the body cells distributed through blood circulation.
2. Variations due to fever Fever is a regulated elevation of the set point of core temperature in the hypothalamus, caused by circulating pyrogens produced by the immune system. A rise in core temperature due to fever may result in feeling cold in an environment that people without fever do not.
3. Variations due to women's menstrual cycles

Thermoregulation is one of the extremely important part in maintaining health, because human life is only compatible with a limited range of temperatures. Core temperature changes of the order of 3°C will not interfere with physical functions, but any variation outside that range has very serious effects.
For example, at 28°C, the muscles can no longer respond. At 30°C, confusion occurs and the body can no longer control its temperature. At 33°C, loss of consciousness takes place.
Hypothermia, cold and freezing environment temperature can cause serve damage to the human body. Cold weather can reduce the core body temperature below 37°C. Frost bite occurs resulting in frozen body tissue, death can also occur.

At 42°C, the CNS(Central Nervous System) breaks down with irreversible brain damage. Death occurs as the result of the body denatured protein

Humans are the most prolific sweaters in the entire animal kingdom

Sweating is accomplished through specialized eccrine sweat glands These glands are found in the dermis and epidermis, distributed all over the body, except for the margins of the limbs, sex organs, and ear drums.

The sweat glands are innervated by the sympathetic nervous system.

When a rise in core temperature is detected by the hypothalamus, it will send impulses to the sympathetic system which would cause an increase in sweat output.

1. Hairs on the skin trap more warmth if they are standing up, and less if they are lying flat. Tiny muscles in the skin can quickly pull the hairs upright to reduce heat loss, or lay them down flat to increase heat loss.

2.Glands under the skin secrete sweat onto the surface of the skin in order to increase heat loss by evaporation if the body is too hot. Sweat secretion stops when body temperature returns to normal.

3. Blood vessels supplying blood to the skin can swell or dilate (called vasodilation) - so that more heat is carried by the blood to the skin where it can be lost to the air; or shrink down again (called vasoconstriction) - to reduce heat loss through the skin once the body temperature has returned to normal.

karen hand with goosebumps and xiufang hand in normal state

The sympathetic nervous system directs the body’s attempts to regulate temperature by rerouting circulation and sweating
Red skin indicates vasodilation and the pooling of blood near the surface for release of heat
Sweating causes evaporative cooling

The individual reduces heat stress by lowering physical activity levels, scheduling work during cool times of the day, wearing less clothes, using fans and air conditioning.

The body attempts to increase and conserve body heat by rerouting circulation and shivering
Vasoconstriction causes the blood to pool internally to conserve organ heat
Shivering causes the temperature to increase due to muscular activity
Individuals respond to cold stress by increasing muscular activity, wearing more clothes, or heating their living space

1. Sweat stops being produced.
2. The minute muscles under the surface of the skin called erectorpili muscles contract, lifting the hair follicle upright. This makes our hairs stand on end which acts as an insulating layer, trapping heat.
This is what also causes goose pimples since humans don't have very much hair and the contracted muscles can easily be seen.
3. Arterioles carrying blood to superficial capillaries under the surface of the skin can shrink blood is rerouted away from the skin and towards the warmer core of the body. This prevents blood from losing heat to the surroundings and also prevents the core temperature dropping further. This process is called vasoconstriction. In extremely cold conditions excessive vasoconstriction leads to numbness and pale skin. Frostbite only occurs when water within the cells begins to freeze.
4. Muscles can also receive messages from the thermo-regulatory centre of the brain (the hypothalamus) to cause shivering. This increases heat production as respiration is an exothermic reaction in muscle cells. Shivering is more effective than exercise at producing heat because the animal remains still.

Ways of How Heat Transfer

Conduction is the process of losing heat through physical contact with another object or body. For example, if you were to sit on a metal chair, the heat from your body would transfer to the cold metal chair.

Convection is the process of losing heat through the movement of air or water molecules across the skin. Example is the use of a fan to cool off the body. The amount of heat loss from convection is dependent upon the airflow or in aquatic (in water) exercise, the water flow over the skin.

Radiation is a form of heat loss through infrared rays. This involves the transfer of heat from one object to another, with no physical contact involved. For example, the sun transfers heat to the earth through radiation.
The last process of heat loss is evaporation.

Evaporation is the process of losing heat through the conversion of water to gas (evaporation of sweat). The primary heat loss process for aqua enthusiasts is convection, however, in an outdoor pool on hot day evaporation will also play a primary role in heat loss.

Physiological temperature regulation in vertebrates

Ectotherms
Even though some fishes are ectotherms they have developed the ability to remain functional even when the water temperature is below freezing and some even use natural antifreeze to resist ice crystal formation in their tissues; amphibians (also ectotherms) must cope with the loss of heat through their moist skins by evaporative cooling.

Ectothermic cooling
Vaporisation: getting wet in a river , lake or sea
Convection: Entering a cold water or air cuttent
Conduction: Lie on cold ground , staying wet in river
Radiation : Find shade , Expose wing surfaces
Ectothermic heating
Convection: Entering a warm water
Conduction : lie on hot rock
Radiation: Lie under the sun

Endotherms
Birds avoid overheating by panting, unlike the mammals, their thin skin has no sweat glands.
Down feathers trap warm air acting as excellent insulators which is used by humans in winter wear.
Hair in mammals also acts as a good insulator; mammalian skin is much thicker than that of birds and often has a continuous layer of insulating fat beneath the dermis.

Heat production in birds and mammals
In cold environments, birds and mammals can compensate for heat loss by:
1. utilizing small smooth muscles which are attached to feather or hair shafts; this shivering thermogenesis distorts the surface of the skin as the feather/hair shaft is made more erect (called goose flesh or pimples).
2. animals in cold climates are tentatively to be larger (easier to maintain core body temperature) than similar species in warmer climates.
3. capable of storing energy as fat for metabolism
4. some have countercurrent blood flow in extremities (e.g. timber wolves) to avoid freezing of tissues.

In warm environments, birds and mammals avoid overheating by:
1. behavioral adaptations like living in burrows during the day.
2. evaporative cooling by perspiration and panting.
3. storing fat reserves in one place (e.g. camel's hump) to avoid its insulating effect.

Behavioral temperature regulation
Some animals living in cold environment maintain their body temperature, preventing heat loss. They let their fur grow more to increase the insulation. Some arctic animals allow their body extremities to cool to very low temperature. Compared to the core body temperature, their legs or nose are extremely low nearly zero celsius, so they have nothing to lose heat in legs or nose. Because the extremities are not insulated well, high temperature in foot and hooves is hard to maintain.

Hibernation estivation

Rather than cope with limited food resources and low temperatures, some mammals hibernate in underground burrows; in order to remain in "stasis" for long periods.
These animals must build up fat reserves and are capable of slowing all body functions.
Some hibernators (e.g. groundhogs) keep their body temperature down throughout their hibernation while the others (e.g. bears) varies with them sometimes emerging from their dens for brief periods.
Estivation occurs in summer and allows some mammals to survive periods of high temperature and little water (e.g. turtles burrow in pond mud).

Conclusion

The body’s ability to adapt to heat and cold are crucial to the maintenance of life. The mechanisms described above are capable of controlling the temperature within defined limits. If external factors become too extreme the body cannot compensate and death may result.

How does Thermoregulation link to Physics then?
The physics of heat transfer

The four avenues by which heat is transferred between the body and the environment are conduction, convection, radiation and evaporation.
The rates of conduction, convection and radiation depend on the temperature gradient between the skin and the environment.
When skin temperature is higher than the atmosphere temperature, heat will flow from the skin to the environment. In contrast, when the environment is cooler than the skin, heat will flow from the body to the environment.
The rate of evaporative cooling depends on the amount of fluid available for evaporation (primarily sweat in humans).

1. The humidity of the environment (the more humid it is, the slower the evaporation).
2. Wind velocity (movement of air over the skin enhances evaporation).

The Rate of heat production during exercise is proportional to the body mass. This means the more body mass one have, the more heat one produces during exercise.
The rate of heat dissipation through each of the four avenues (as mentioned above) depends on the surface area of the skin.

A person’s surface area-to-mass ratio, therefore, affects the ratio of heat dissipation to heat production in both hot and cold climates. This surface area-to-mass ratio is important in understanding the effects of growth on thermoregulatory effectiveness, because the smaller the person is , the larger the surface-to-mass ratio. An example is shown below. Despite the child whom is small in size, he has a larger surface-to-mass ratio (380cm2/kg) as compared to the adult.

As a result, irrespective of physiological processes, the rates of heat conduction, convection ad radiation between the child and the environment would be faster than the adult. Likewise, even if the sweating rate per unit of surface area is similar in children and adults, the rate of evaporation per kg body mass would be faster in the child.